arm64: KVM: Move __cpu_init_stage2 after kvm_call_hyp

In order to ease the merge with the rest of the arm64 tree, move the
definition of __cpu_init_stage2() after what will be the new kvm_call_hyp.
Hopefully the resolution of the merge conflict will be obvious.

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>